(3) بَابُ السَّلاَمُ اسْمٌ مِنْ أَسْمَاءِ اللَّهِ تَعَالَى
Narrated `Abdullah:
When we prayed with the Prophet (ﷺ) we used to say: As-Salam be on Allah from His worshipers, As- Salam be on Gabriel, As-Salam be on Michael, As-Salam be on so-and-so. When the Prophet (ﷺ) finished his prayer, he faced us and said, "Allah Himself is As-Salam (Peace), so when one sits in the prayer, one should say, 'at-Tahiyatu-li l-lahi Was-Salawatu, Wat-Taiyibatu, As-Salamu 'Alaika aiyuhan- Nabiyyu wa Rah-matul-iahi wa Barakatuhu, As-Salamu 'Alaina wa 'ala 'Ibadillahi assalihin, for if he says so, then it will be for all the pious slave of Allah in the Heavens and the Earth. (Then he should say), 'Ash-hadu an la ilaha illalllahu wa ash-hadu anna Muhammadan `Abduhu wa rasulu-hu,' and then he can choose whatever speech (i.e. invocation) he wishes " (See Hadith No. 797, Vol. 1).
Reference: Sahih al-Bukhari 6230
In-book reference: Book 79, Hadith 4
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 74, Hadith 249